Balanced dynamic compressor amplifier



y 1951 E. s. PURINGTON 2,553,674

BALANCED DYNAMIC COMPRESSOR AMPLIFIER Filed April 28, 1943 3nvcntor ELLIS ON S. PURINGTON Cittorneg Patented May 22, 1951 BALANCED DYNAMIC COMPRESSOR AMPLIFIER Ellison S. Purington, Gloucester, Mass., assignor to John Hays Hammond, Jr., Gloucester, Mass.

Application April 28, 1948, Serial No. 23,663

1 Claim. 1

This invention relates to compressor circuits for compressin the dynamic range of the signal in the output circuit of an electronic amplifier and ha for an object to provide a circuit of the above type having novel and improved operating characteristics.

Another object is to provide a circuit of the above type having improved efiiciency of operation and in which the thump eifect and the efiect of the control voltage are eliminated.

Various other objects and advantages will be apparent as the nature of the invention is more fully disclosed.

In accordance with the present invention the single-ended dynamic compressor amplifier includes a pair of electron tubes with plates and cathodes connected in parallel, with a common plate feed impedance and a common external output circuit. The input signal is impressed upon the grids of the two tubes in phase opposition but the grid circuits of the two tubes are differently biased so that the outputs are normally unbalanced and an output signal is obtained due to the differential efiect of the two outputs. Control voltages are derived from the common out put circuit by means of a rectifier network and are applied to the grid return paths of the two tubes in opposite senses so that an increased signal serves to decrease the conductance and plate :current of one tube, but increases the conductance and plate current of the other tube in such a manner that the unbalance is decreased with increased signal. Hence the overall gain from source to output is a decreasing function of the signal.

changes of space currents in the two tubes are in opposite senses. In addition the dynamic tubes have substantially the same bias and plate currents, approaching class A operation, when the signal through the tubes is a maximum.

The novel features which are characteristic of this invention will be better understood by referring to the following description taken in con nection with the accompanying drawings in which a specific embodiment thereof has been set forth for purposes of illustration.

In the drawings the figure is a schematic diagram of a dynamic compressor system embodying the present invention.

In the embodiment shown a pair of dynamic amplifier tubes, indicated as tubes In, II have their plates lila, I la connected together and connected through a feed resistor l2 to the positive end of battery l3, the negative end of which is This arrangement provides for thump and transient reduction because the 2 grounded at I4. The cathodes lllc, lie of tubes to, i! are also connected together and are connected through cathode resistors l5 and i6, bypassed by capacitors l'! and it, to ground at l9. A bleeder resistor is connected from the positive side of battery [3 to the cathodes of tubes Ill, H. One input terminal 2| of the entire system is connected through a blocking capacitor 22 to the grid 23g of a phas inverter tube 23. The other input terminal 24 is connected to ground lead 25. The cathode 230 is connected to ground lead 25 through a self-biasing resistor 26. A resistor 27 is connected across the input circuit. The plate 23a of the tube 23 is connected through a resistor 28 to the positive side of the battery l3, and through a blocking capacitor 29 to the grid lily of triode Ill. The grid mg is connected t the junction of resistors l5 and I6 through resistor 30 from which it receives its signal voltage, and a resistor 3! from which it receives its automatic control voltage. The resistors 3!! and 3| are so related to the resistors l5 and to that for weak signals the grid of tube I0 is biased positive with respect to ground but not to cathode potential.

The cathode 230 of phase inverter tube 23 is connected through a blocking capacitor 34 to the grid H g of tube II. The grid Hg is connected to the grounded end of resistor l6 through a resistor 35 from which it receives its ignal voltage and a resistor 36 from which it receives its automatic control voltage. These resistors are so related that for weak signals the grid Hg is more negative than the grid Ilia. The control :currents in the resistors 35 and 36 are so poled that the signal increases the negative bias of grid lug and decreases the negative bias on grid Hg and the bias values approach each other as the signal is increased. Because of the phase reversal effect of thesignal by the tube 23, the overall gain of the system is diminished as these changes of bias occur. I I

The plates [9a, Ila of tubes l0 and H are connected to ground through blocking capacitor 4!! and a volume control potentiometer 4!. Output terminals 42 and t3 are connected respectively to a variable tap 44 of the potentiometer M and ground. To provide for automatic change of bias of the tubes l6 and il, a dynamic control potentiometer 45 is provided, one end of which is grounded, the other end of which is coupled through a capacitor 46 to the arm of a double throw switch 41 having two fixed contacts 46 and 49 connected to the grid Hg and the plate Ila respectively. The tap 50 of the potentiometer 45 is connected by a lead to the grid 52g of a rectifier driver tube 52, the cathode 520 of which is connected to ground through a resistor 53 shunted by a capacitor 54. The plate 52a of tube 52 is connected through a feed resistor 55 and decoupling resistor 56 to the positive end of battery |3. The junction of resistors 55 and 56 is connected to ground by bypass capacitor 5'! and is connected through bleeder resistor 58 to the cathode 520.

The plate 52a of the rectifier driver tube 52 is connected through capacitor 66 to the cathode of a rectifier 6|, the anode of which is connected through a resistor 62 to the grid side of resistor 3|. The anode and cathode of the rectifier 6| are connected respectively by resistors 63 and 64 to the cathode side of resistor 3|. Resistors 3| and 63 are bypassed by capacitors 55 and 66 respectively.

Similarly the plate 52a of the rectifier tube 52 is connected through capacitor 10 to the anode of a rectifier H the cathode of which is connected through a resistor 12 to the grid side of resistor 36. The anode and cathode of the rectifier H are connected through resistors 13 and M to the cathode side of resistor 36. Resistors 36 and 14 are bypassed by capacitors l5 and 78 respectively.

In the operation of this system for weak signals the voltage on the grid |0g is substantially equal and opposite to the voltage on the grid ||g, but tube It is more efiective in producing amplified output than tube due to its lesser negative bias. Therefore weak signals are amplified well. However, as the signal increases, the rectifier driver tube 52 is actuated by the output signal derived from the potentiometer 45, which automatically changes the bias on the grids of tubes l0 and II in such, a fashion that the difierence in amplifying ability is reduced, and the gain of the system as a whole for strong signals is less than for weak signals. The constants of the rectifier system are so chosen that the control of bias is substantially in accordance with the acoustical value of the output signal; so that the rectified outputs of devices 6| and H are not excessive; so that the D. C. component of the current through feed resistor I2 is substantially independent of signal level; and so that the amplification produced by the system follows the increase and decrease of signal level in a smooth manner.

If desired, the switch 4'! may be closed to the contact 48 so that the gain of the amplifier may be controlled by the acoustic level of the input signal instead of the acoustic level of the output signal. Limiters may be used and so adjusted that for strong signals the gain of the two tubes may approach cancellation to any desired degree. The shape of the curve relating output decibels to input decibels may be controlled by design to meet various requirements, by use of limiters and by use of over biasing the rectifier driver 52 or by biasing the rectifiers 6| and H. Various other modifications may be made by those skilled in the art.

Although only a few of the various forms in which this invention may be embodied have been shown herein, it is to be understood that the invention is not limited to any specific construction but might be embodied in various forms without departing from the spirit of the invention or the scope of the appended claim.

What is claimed is:

A compressor amplifier system comprising a pair of electronic tubes having plates, cathodes and control grids, said plates bein connected together and said cathodes being connected together to provide parallel space current paths, a common output circuit connected between said plates and a point of fixed reference potential, a

source of direct-current plate voltage having a positive and a negative terminal, said negative terminal being connected to said point of fixed reference potential, a common feed resistor connected between said plates and said positive terminal, a common cathode impedance comprising two series connected cathode resistors connected between said cathodes and said point of fixed reference potential, an input circuit including a phase inverter tube having a plate, a control grid and a cathode, said phase inverter plate being connected to the grid of the first of said pair of electronic tubes, said phase inverter cathode being connected to the grid of the second of said pair of electronic tubes, whereby to impress signals upon said control grids in phase opposition, a first pair of serially connected grid resistors connected between the control grid of the first of said pair of electronic tubes and the common connection of said cathode resistors, a second pair of serially connected grid resistors connected between the control grid of the second of said pair of electronic tubes and said point of fixed reference potential, whereby difierent biasing potentials are applied to the respective electronic tube control grids, a rectifier driver amplifier circuit having an input circuit and an output circuit, said rectifier driver input circuit being connected to said common output circuit and including selective switching means responsive to the signal voltage to actuate said rectifier driver, a pair of rectifiers, said rectifiers being connected to said rectifier driver output circuit in oppositely poled relation, and a resistor-capacitor network connected with each of said rectifiers, one of said networks being connected with one of said grid resistors of said first pair of serially connected grid resistors, the other of said networks being connected with one of said grid resistors of said second pair of serially connected grid resistors, whereby control voltages are applied to said shunt connected tubes in opposite sense so that an increase in control voltage decreases the difference in bias on the respective grids whereby the output signals are brought more nearly into balance and the amplification of the system is decreased withv increased. input, signal level.

ELLISON S. PURINGTON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,251,594 Mayne Aug. 5, 1941 2,368,453 Deerhake Jan. 20, 1945 Shimek Jan. 24, 1950 

